1. Technical Field
The present invention relates to a fringe field switching (FFS) mode liquid crystal display device and a method for manufacturing such a device. More particularly, the invention relates to an FFS mode liquid crystal display device and a method for manufacturing such a device, in which it is not only possible to simplify the layered structures of a display area and a peripheral circuit formed on a substrate thereof, but also to decrease manufacturing costs thereof while guaranteeing moisture resistance equivalent to that of the related art, and in which a variation in the characteristics of the TFTs of the peripheral circuit is small, thus guaranteeing a highly reliable operation of such a device.
2. Related Art
Many of the liquid crystal display devices being used are of the vertical electric field type which has a pair of transparent substrates with electrodes and the like formed on their surfaces and a liquid crystal layer that is sandwiched between the pair of substrates, and displays various kinds of information by rearranging the liquid crystals via application of a voltage to the electrodes on the two substrates. Such vertical electric field type liquid crystal display devices are generally of a twisted nematic (TN) mode, but since this mode has the problem of a narrow viewing angle, various improved vertical electric field type liquid crystal display devices of a vertical alignment (VA) mode, a multi-domain vertical alignment (MVA) mode or the like have been developed.
On the other hand, liquid crystal display devices that, unlike the vertical electric field type liquid crystal display devices described above, are of an in-plane switching (IPS) mode or a fringe field switching (FFS) mode, in which a pair of electrodes constituted of a pixel electrode and a common electrode are provided on only one of the substrates, are also known.
Of these, the IPS mode liquid crystal display device disposes the pair of electrodes on the same layer and applies the electric field to the liquid crystals in a direction almost parallel to the substrates, thus rearranging the liquid crystal molecules in a direction parallel to the substrates. Therefore, the IPS mode liquid crystal display device is also called a transverse electric field type liquid crystal display device, and has the advantage of providing a viewing angle that is extremely broad in comparison to the vertical electric field type liquid crystal display devices described above. With the IPS mode liquid crystal display device, however, because the pair of electrodes are provided on the same layer in order to apply an electric field to the liquid crystals, there is the problem that the liquid crystal molecules, which are positioned above the pixel electrode, are inadequately driven, and this results in lowering of transmissivity and other aspects.
What might be termed diagonal electric field type FFS mode liquid crystal display devices have been developed in order to resolve the problems of the IPS mode liquid crystal display device (see JP-A-2001-235763 and JP-A-2002-182230). In such FFS mode liquid crystal display devices, the pixel electrodes and common electrodes for applying the electric field to the liquid crystal layer are disposed in different layers, each with an insulating film (hereinafter referred to as “inter-electrode insulating film”) interposed therebetween.
Such an FFS mode liquid crystal display device has a broader viewing angle and higher contrast than the IPS mode liquid crystal display device, and can be driven with low voltage as well as having higher transmissivity. Thus it has the feature of being able to achieve a bright display. In addition, the FFS mode liquid crystal display device has the advantage that the area of overlap, viewed from above, between the pixel electrodes and the common electrodes, is larger than that in the IPS mode liquid crystal display device. As a result, a larger auxiliary capacitance is generated and there is no need to specially provide an auxiliary capacitance wire, which enables a higher aperture ratio to be obtained than that in the IPS mode liquid crystal display device.
However, the FFS mode liquid crystal display devices disclosed in JP-A-2001-235763 and JP-A-2002-182230 has the problem that, because there is misalignment of the liquid crystal molecules due to a potential difference between the signal line and the pixel electrode, portions at the vicinity of the signal line constitute regions that do not contribute to the display and therefore the aperture ratio decreases. Another problem is that there is a coupling capacitance between the signal line and the pixel electrode, which causes deterioration in display quality such as crosstalk. An approach that is practiced to address such problems is to employ an interlayer resin film such as is used in the above-mentioned vertical electric field type liquid crystal display devices, and to dispose the pixel electrodes, the common electrodes and the like on the interlayer resin film, in order to eliminate the influence of the potential of the signal line (see JP-A-2001-083540 and JP-A-2007-226175).
In the liquid crystal display device of the related art, peripheral circuits comprising thin film transistors (TFTs), which are constituted of an electrostatic protection circuit, a driver circuit, a logic circuit, and the like, are formed on the peripheral portion of the display area, and a passivation film formed of silicon nitride is formed so as to cover the channel regions of the TFTs of the peripheral circuits as well as the channel regions of the TFTs used as the switching elements of the display area. The passivation film is provided in order to guarantee the moisture resistance of the channel regions of the TFTs of the peripheral circuits as well as the channel regions of the TFTs of the display area. In the case where the interlayer resin film is formed to provide an improved aperture ratio as in the liquid crystal display device disclosed in JP-A-2001-083540 and JP-A-2007-226175, the interlayer resin film is formed on the surface of the passivation film, similar to the interlayer resin film provided in the vertical electric field type liquid crystal display device disclosed in JP-A-2001-235763 and JP-A-2002-182230.
That is to say, in the vertical electric field type liquid crystal display device as disclosed in JP-A-2001-235763 and JP-A-2002-182230, the channel regions of the TFTs of the peripheral circuits as well as the channel regions of the TFTs of the display area, which are used as the switching elements, are covered with an insulating film comprising two layers of a passivation film and an interlayer resin film. On the contrary, in the case of the transverse electric field type liquid crystal display device as disclosed in JP-A-2001-083540 and JP-A-2007-226175, the channel regions of the TFTs of the peripheral circuits as well as the channel regions of the TFTs of the display area, which are used as the switching elements, are covered with an insulating film comprising two layers of a passivation film and an interlayer resin film, similar to the vertical electric field type. Moreover, the channel regions are also covered with the inter-electrode insulating film provided between the pixel electrode and the common electrode, and are therefore covered with an insulating film comprised of at least three layers. Therefore, the liquid crystal display device disclosed in JP-A-2001-083540 and JP-A-2007-226175 can reliably provide improved aperture ratio but many layered structures have to be formed on one transparent substrate, so that more manufacturing processes are needed and the manufacturing costs will increase.
Inventors of the application have been persistently diligent in their investigation of the structure of the transverse electric field liquid crystal display device disclosed in JP-A-2001-083540 and JP-A-2007-226175. As a result of the studies, the inventors have discovered that, since the inter-electrode insulating film and the passivation film are formed of the same silicon nitride, sufficient moisture resistance can be secured when the passivation film was not provided and the channel regions of the TFTs of the display area, which are used as the switching elements, were covered with only the interlayer resin film and the inter-electrode insulating film. Such findings have already been filed for patent application (Japanese Patent Application No. 2007-309978; hereinafter referred to as “Prior Application”). Since the liquid crystal display device of Prior Application does not have the passivation film for the TFTs of the display area, the passivation film is also not provided for the TFTS of the peripheral circuits formed on the peripheral portion of the display area so that the channel regions are directly covered with the inter-electrode insulating film.
In the liquid crystal display device of Prior Application, the channel regions of the TFTs of the peripheral circuit are covered with the inter-electrode insulating film by the following steps.
A first step in which similar to the TFTs of the display area, a gate electrode, a gate insulating film, a semiconductor layer, a source electrode and a drain electrode are formed on a TFT forming region of the peripheral circuit which is formed on a peripheral portion of the display area, thus forming TFTs in which the channel regions are exposed.
A second step in which an entire surface of the substrate obtained in the first step is covered with an interlayer resin film, contact holes are formed on the interlayer resin film so as to expose the drain electrodes of the TFTs of the display area, and the interlayer resin film on the peripheral portion of the display area is removed.
A third step in which the entire surface of the substrate obtained in the second step is covered with a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide), which is then etched so as to form a lower electrode on the surface of the interlayer resin film of the display area.
A fourth step in which the entire surface of the substrate obtained in the third step is covered with an inter-electrode insulating film formed of silicon nitride or silicon oxide.
In the second step, the interlayer resin film is formed on the display area by a photolithographic method using a photoresist material, and at this time, a developing process, a bleaching process, and a firing process are performed. In the third step, after the transparent conductive material is covered, the transparent conductive material is patterned by photolithographic and etching methods. In the fourth step, the inter-electrode insulating film is formed by, for example, a CVD method.
Therefore, in the second step of the liquid crystal display device of Prior Application, the channel regions of the TFTs of the peripheral circuit are exposed to a developing solution during the photolithographic process of the interlayer resin film and to the atmosphere during the bleaching and firing processes. Moreover, in the third step, the channel regions are exposed to a cleaning solution during the cleaning before the film is formed and are exposed to a resist peeling solution after the film is formed. Furthermore, in the fourth step, the channel regions are exposed to a cleaning solution during the cleaning before the film is formed and are always exposed to the atmosphere until the film forming of the inter-electrode insulating film is completed.
Therefore, in the liquid crystal display device of Prior Application, since it is difficult to prevent the channel regions of the TFTs in the peripheral circuits of the display area from being exposed to various chemical solutions until they are covered with the inter-electrode insulating film, the damage to the channel portion will increase, thus increasing the variation in the characteristics of the TFTs of the peripheral circuits and impairing the reliability thereof.